1. Field of the Invention
This invention relates to semiconductor fabrication. More specifically, the present invention relates to the use of reactive ion etching in semiconductor fabrication.
2. Discussion of the Related Art
Light emitting diodes, commonly referred to, as xe2x80x9cLEDsxe2x80x9d are well-known semiconductor devices that convert electrical current into light. The color of the light (wavelength) emitted by an LED depends on the semiconductor material that is used to fabricate the LED. This is because the wavelength of the emitted light depends on the semiconductor material""s band-gap energy, which represents the energy difference between valence band and conduction band electrons.
Gallium-Nitride (GaN) has recently gained much attention from LED researchers because GaN has a band-gap energy that is suitable for emitting blue light. Blue light emitting LEDs are important because of the short wavelength of blue light, which is beneficial in applications such as optical recordings, and because of the possibility of producing a wide range of colors when used with red and green LEDs. Accordingly, GaN technology has been and continues to be rapidly evolving. For example, the efficiency of GaN LEDs has surpassed that of incandescent lighting. Thus, the market growth for GaN-based LEDs is rapid.
Despite the evolution of GaN technology, GaN-based devices are too expensive for most applications. One reason for this is the high cost of manufacturing GaN-based devices, which in turn is related to the difficulty of growing GaN epitaxial layers and then processing GaN devices grown on hard substrates, such as sapphire or silicon carbide.
High quality GaN epitaxially grown layers are typically fabricated on sapphire substrates. This is because sapphire lattice matches well with GaN. Furthermore, the sapphire crystal is chemically and thermally stable, has a high melting temperature, a high bonding energy (122.4 Kcal/mole), and a high dielectric constant. Chemically, sapphires are crystalline aluminum oxide, Al2O3.
Despite sapphire""s numerous advantages, it has significant problems. For example, sapphires are extremely hard, have a crystal orientation without natural cleave angles, and are thus difficult to dice and mechanically polish (process steps that greatly assist the production of low-cost, high quality devices). Furthermore, sapphire""s high bonding strength results in a chemical makeup that is resistant to wet chemical etching. As a result, sapphire requires special processing techniques when used as a device substrate.
Fabricating semiconductor devices on sapphire is typically performed by growing GaN epitaxial layer on a sapphire substrate using MOCVD (Metal Organic Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy). Then, a plurality of individual devices, such as GaN LEDs, are fabricated on the epitaxial layer using normal semiconductor processing techniques.
After the individual devices are fabricated the individual devices must be separated (diced) from the sapphire substrate. To do this the sapphire substrate is first mechanically ground, lapped, and/or polished to produce a thin wafer having a smooth backside. It should be noted that such mechanical steps are time consuming and expensive. After thinning and polishing, the sapphire substrate is attached to a supporting tape. Then, a diamond saw or stylus forms scribe lines between the individual devices. Such scribing typically requires at least half an hour to process one 2xe2x80x3 substrate (wafer), adding even more to manufacturing costs. Additionally, since the scribe lines have to be relatively wide to enable subsequent dicing, device yields are reduced, adding even more to manufacturing costs. After scribing, the sapphire substrates are rolled using a steel roller, or applied to a shear cutting process, to produce stress cracks that subsequently dice or separate the individual semiconductor devices.
Because of cost considerations, in practice it is highly beneficial to process more than one substrate at a time. However, doing this by mechanical lapping and scribe line cutting is not currently practical. Thus, the mechanical work processes increase cost simply because each substrate must be individually worked. Furthermore, mechanical work processes tend to reduce yield simply because of the handling steps that are required.
Thus, while highly beneficial in many aspects, sapphire substrates have serious problems. Therefore, a new method of separating devices fabricated on sapphire substrates, or in general, on any other substrate, would be beneficial. Even more beneficial would be a new method of dicing devices with fewer mechanical handling steps. Such methods would be particularly useful if they enable increased device yield. Methods that also enable simultaneous processing of multiple substrates would be particularly useful. Also, a new method of dicing sapphire substrates at relatively fast speeds along thin, accurately controlled dice lines, and with minimal mechanical steps would be particularly beneficial. Furthermore, a non-mechanical method of thinning sapphire substrates would be particularly advantageous.
The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole.
The principles of the present invention provide a new method of dicing substrates to separate out individual semiconductor devices that were fabricated on the substrate. By using these principles, the number of acceptable individual semiconductor devices (chips) from some substrates can be increased, thus enhancing the productivity of the semiconductor fabrication process. The principles of the present invention are particularly useful when separating semiconductor devices, such as GaN LEDs, that are fabricated on extremely hard substrates, such as sapphire and silicon carbide.
A method of dicing substrates to separate individual semiconductor devices according to the principles of the present invention includes the step of forming a mask pattern on a substrate or on the device-side surface. The mask pattern includes scribe lines that expose the substrate or device-side surface for etching. Such a mask pattern is beneficially produced using photolithographic techniques and subsequent development. Beneficially, the mask pattern is comprised of a relatively thick, hard photo-resist, a hard metal mask (such as Cr, Mo, etc.), or a combination of metal and photo-resist. Then, the substrate or device-side surface is etched along the scribe lines using inductively coupled plasma reactive ion etching (ICP RIE). The etching gas is comprised of BCl3 and/or BCl3/Cl2, possibly with Ar added. The etched result creates trenches that extend into the substrate. Then, the etched substrate is stress processed. The stress process produces stress lines that extend from the trenches through the substrate. The stress lines cause the substrate to separate in a controlled manner so as to separate the individual semiconductor devices. The stress can be applied in numerous ways, such as by applying a supporting tape that holds the substrate and then rolling a roller across the back of the supporting tape, or by forcing a knife edge toward the trenches. Beneficially, most of the processing steps can be performed simultaneously on a plurality of substrates.
The principles of the present invention are particularly useful when separating semiconductor devices fabricated on extremely hard substrates such as sapphire or SiC. Other substrates that can be used include Si, GaAs, InP, ZnSe, ZnO, and GaP. The principles of the present invention can reduce the number of mechanical handling steps required while enabling simultaneous processing of multiple substrates. Furthermore, the principles of the present invention enable fast dicing of the hard substrate, with the dicing occurring along thin, accurately controlled scribe lines and with minimal mechanical working.
A method of dicing hard substrates according to the principles of the present invention includes forming a mask pattern (see above) on a hard substrate, with the mask pattern having scribe lines that expose the substrate or the device-side surface. Then, the substrate or device-side surface is etched along the scribe lines using inductively coupled plasma reactive ion etching (ICP RIE) with an etching gas comprised of BCl3 and/or BCl3/Cl2, possibly with Ar added. Etching produces trenches that extend into the hard substrate. Then, the hard substrate is stressed. Stress processes can be applied by rolling, use of a knife-edge, or other suitable means. The resulting stress process produces stress lines that extend from the trenches through the hard substrate. The hard substrate can then be diced along the stress lines to separate individual devices.
Beneficially ICP RIE is performed such that the trenches are formed with notches at the bottom tip of the trenches. Such notches readily enable cleaving along the stress lines.
In addition, the principles of the present invention provide for methods of polishing a substrates using inductively coupled plasma reactive ion etching (ICP RIE), with the ICP RIE gas being BCl3 and/or BCl3/Cl2, possibly with Ar added.
The novel features of the present invention will become apparent to those of skill in the art upon examination of the following detailed description of the invention or can be learned by practice of the present invention. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating certain embodiments of the present invention, are provided for illustration purposes only because various changes and modifications within the spirit and scope of the invention will become apparent to those of skill in the art from the detailed description of the invention and claims that follow.